My guess is that you have a ground fault--signal generator common connected to scope common--isolate ground pin on one unit and see what happens--or connect scope as a differential input, if it has that feature

You could do it using a source which is isolated, perhaps by passing the signal through an isolating transformer.

Alternatively, you might do a differential measurement with two inputs subtracted, if the Tek 2205 scope supports that function. Edit: it seems likely that it does: there is an "ADD" vertical display option and Y channel 2 can be inverted.

NB for correct differential mode operation, the Y channel sensitivity settings must be equal. Before engaging "ADD", but with Y2 already inverted, ensure that both channel displays are entirely on the screen.

The problem is that the output of your signal generator is grounded too. So it's connecting one node of your bridge to ground, and the scope is connecting a different node to ground.

In most applications of full wave bridge rectifier circuits (power supplies), the AC drive to the bridge is coming from a transformer - neither of the two connections are grounded. If you transformer couple your signal generator to the bridge, it will work fine.

The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.

The problem is that the output of your signal generator is grounded too. So it's connecting one node of your bridge to ground, and the scope is connecting a different node to ground.

In most applications of full wave bridge rectifier circuits (power supplies), the AC drive to the bridge is coming from a transformer - neither of the two connections are grounded. If you transformer couple your signal generator to the bridge, it will work fine.

The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.

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Hi thanks for the idea

So instead of using two channels i just made a common ground instead , because u said it was connect to a different ground.

That did not work of course or im sure u would have suggested it.
Why is that

Also so i set up the scope like u suggested but can u explain what goes on channel 2

So instead of using two channels i just made a common ground instead , because u said it was connect to a different ground.

That did not work of course or im sure u would have suggested it.
Why is that

Click to expand...

I don't really know what you mean by that, but whatever ground arrangements you try to make, the mains ground is still there. (And it has to stay there - don't even think of trying to disconnect it as this is extremely dangerous. )

Try drawing the circuit on paper, including all the ground connections. The cause of the short-circuits will probably become clear to you.

One probe goes to the anode of the bridge (+), while the other probe goes to the cathode (-).
Both channels must be set at the same sensitivity and channel 2 must be inverted.
The scope probe ground leads are not connected in this case.

Your function generator most likely has a coaxial output that you are connecting to the AC ports on the bridge. The shield connection of the coaxial output of the generator is GROUND. Thus, you are connecting GROUND to one side of the AC inputs of the bridge.

The scope inputs are coaxial also. The shield connections of the CH1 and CH2 are also GROUND. Connect these GROUNDS to the same exact place as that you connected GROUND of the function generator. The center connections of CH1 and CH2 then connect to the + and - ports of the bridge. Then, invert CH2 and select the ADD vertical mode (both are settings on the scope)

I don't know how to make it clearer than that.

Of course, this will only work IF the function generator swings above and below GROUND.

The other thing you can do is make the scope measurement differentially. Use two inputs to the scope. Use channel one to measure between ground and the + output of the bridge, and CH2 to the - output of the bridge. Then, set the scope to invert CH2, and set the vertical mode to ADD. This will create a pseudo differential measurement across the + and - outputs of the bridge.